My research interest has been focused on the physiological adaptation of animals to their different reproductive states. The oxytocin (OT) neuron in the supraoptic nucleus (SON) of the hypothalamus is one of the best models to study such adaptation, because the importance of OT during parturition and lactation are well documented. The reports of neuronal activity and morphological responses of OT neurons to lactation are abundant as well. However, the mechanisms underlying the physiological adaptation of OT neurons to lactation are only sparsely discussed. OT neurons in the supraoptic nucleus are known to undergo a remarkable change in neuronal activity in response to the increased hormonal demand that occurs during parturition and lactation. OT neurons display a short, high frequency burst proceeding each milk ejection (Poulain & Wakerley, 1982). The bursting activity is synchronized among all OT neurons (Belin et al., 1984) and results in a bolus release of OT into the blood stream, necessary for uterine contraction or contraction of myoepithelial cells in the mammary glands. This pulsatility is believed to maximize the biological effects of OT and allows the neurons to recover from secretory fatigue (Bicknell, 1988). Because of their significance in controlling hormone secretion, mechanisms involve in the regulation of firing rate and pattern in OT neurons are the subject of great interest. The pattern of electrical activity of neurons generally results from the interaction of intrinsic ionic mechanisms and synaptic activity. The intrinsic membrane properties of OT neuron, the spike after-potentials (after hyperpolarizing potentials (AHP) and depolarization after-potential (DAP)), appear to change in favor of the short bursting activity of OT neuron necessary during parturition and lactation. The ionic conductances mediating these properties are, at least partly, involving in the regulation of the firing rate and pattern that ultimately control the release of hormone. Therefore, this application is to elucidate the mechanisms responsible for the changes in physiological properties of OT neurons in response to lactation. The specific aims of the application are to determine 1) what specific component of spike after-potentials (AHPs and DAP) is altered during lactation; 2) whether the responsive component is responsive to changes in the gonadal steroids. [unreadable] [unreadable]